U.S. Pat. No. 4,003,074 discloses an optoelectronics package with an hermetically sealed enclosure containing a semiconductor laser and a monitor of optical emission from a rear facet of the semiconductor laser. A monitor, for example, a PIN diode, in response to the optical emission produces an output voltage for supply to an electrical circuit that controls the current applied to the semiconductor laser. The enclosure incorporates an optical lens for directing rays of optical emission in a narrow course externally of the enclosure. The enclosure itself serves as an alignment fixture for aligning a semiconductor laser and a lens to form an optical coupling. Since the enclosure is of small size, there is a lack of interior space that hinders manipulation of the parts into alignment. Following alignment of the laser and the lens, the enclosure must be hermetically sealed. The alignment of the laser and the lens, achieved with an accuracy measured in microns, can be disrupted when subjected to further process steps that achieve hermetic sealing of the package.
In many applications the emission from the laser must be coupled into an optical fiber. Coupling between an optical fiber and a semiconductor laser is subject to disadvantages and requires a high degree of care in fabrication. A direct attachment of an optical fiber to the laser can be disrupted, because the operating temperature of the laser will fluctuate and cause different dimensional changes respectively in the fiber and the laser. The dimensional changes in response to temperature fluctuation will cause the fiber to change position with respect to the laser or become separated from the laser.
Direct coupling is generally inefficient because of a wide disparity in the dimensions of the guided modes in the laser and in the fiber. This can be overcome by the use of a lens for mode size matching.
An optical coupling is subject to further disadvantages arising from use of a lens to focus optical emission from a laser on an optical fiber. It is difficult to achieve precision optical alignment of the lens and the laser. The fiber can become misaligned due to dimensional changes with temperature fluctuations.